Ionic liquids have received significant attention due in part to their wide potential use and application. The term “ionic liquid” is commonly used for salts whose melting point is relatively low (below 100° C.). Salts that are liquid at room temperature are commonly called room-temperature ionic liquids. Early investigators employed ionic liquids based on dialky-imidazolium salts. For example, Wilkes et. al developed ionic liquids based on dialkly-imidazolium salts for use with an aluminum metal anode and chlorine cathode in an attempt to create a battery. J. Wilkes, J. Levisky, R. Wilson, C. Hussey, Inorg. Chem, 21, 1263 (1982).
Some of the most widely studied and commonly used ionic liquids are based on pyridinium salts, with N-alklypyridinium and N,N′-dialylimidazolium finding significant use. Pyridinium based ionic liquids, including N-alkyl-pyridiums and N,N-dialkylimidazoliums, and nitrogen-based ionic liquids generally posses thermodynamic stabilities limited to 300° C., or less, are readily distillable, and tend to have measurable vapor pressures at temperatures significantly less than 200° C. Such properties limit their usefulness, as well as their applications. For example, such ionic liquids are susceptible to decomposition during back end of line (BEOL) thermal processing. Additionally, such ionic liquids are also decomposed during other heat-transfer processing steps which often subject the ionic liquids to continuous thermal cycling to temperatures exceeding 300° C.
The diverse nature of ionic liquids continues to be explored, and additional uses of ionic liquids have been considered. For example, electrochemical methods and applications are in need of electrolytes to enhance conductivity in a variety of devices and applications. Recent studies have been conducted in the area of room temperature ionic liquids as a possible alternative to conventional solvent based electrolytes.
While developments have been made, it is apparent that a continuing need exists for new developments in ionic liquid compositions and for materials and uses in which ionic liquids may be employed for use in polymer-gel electrolytes in lithium ion batteries, fuel cells, dye-sensitized solar cells and molecular capacitors.